5072 Chemistry Syllabus Notes

Electrolysis

Electrolysis is the conduction of electricity by an ionic compound (an electrolyte), when molten or dissolved in water, leading to the decomposition of the electrolyte.

Implications of electrolysis:

Electrolysis is the evidence for the existence of ions which are held in a lattice when solid but which are free to move when molten or in solution.

Case Study: Electrolysis of sodium chloride

Molten sodium chloride can be electrolyzed to form molten sodium metal and chlorine gas. Molten sodium chloride has mobile sodium and chloride ions that are able to conduct a current. As they conduct the electrical current, the ions are transformed by the free electrons in the electric current to form the electrolysis products.

When electrolyzed using inert electrodes, molten sodium chloride decomposes under the following reactions:

Cathode reaction:Na+ (l) + e- Na (⟶ l)

Anode reaction:

2 Cl- (l) Cl⟶ 2 (g) + 2 e-

Predicting electrolysis products:

The electrolysis of molten binary ionic compounds will form the constituent elements of the compound. Examples include molten sodium chloride (refer to above case study) as well as molten magnesium oxide.

In the electrolysis of aqueous ionic compounds or of non-binary ionic compounds, selective discharge of ions occur according to the electrochemical discharge series.

The order of selective discharge of cations is contingent on the reactivity series of metals. As we move up the reactivity series to the more reactive metals, the ease of discharge is decreased. Thus less reactive metals are selectively discharged before more reactive metals.

The order of discharge of anions is slightly more tricky. As a general rule, the more stable the anion is, the more difficult it is to discharge the ion. As such, these ions will be lower in the discharge series. Nitrate ions for example, are very stable due to the shape of the ion as well as the electronegativity of both nitrogen and oxygen.

An implication of selective discharge is that when some aqueous ionic compounds are electrolyzed, the ionic compound itself does not decompose but rather, water is electrolyzed in its stead. For example, in the electrolysis of dilute sodium chloride solution, sodium chloride itself is not broken down but the water in the solution does; forming hydrogen and oxygen gas.

A factor affecting the products of electrolysis of aqueous ionic compounds is the concentration effect. As an ion becomes more concentrated in a solution, it will collide with the electrode more often, increasing the number of such ions being discharged.

A prominent example of this is with concentrated sodium chloride. As the chloride ions are present in large amounts within the solution, they are able to compete with the hydroxide ions for discharge priority. As such, the anode product of such electrolysis is chlorine gas instead of the typical oxygen gas.

However, this effect is limited by the electrical affinity of the ions present in the solution. For example, chloride ions are able to compete with hydroxide ions when present in a concentrated solution as the ions are not so far apart in terms of their ability to lose electrons (due to the large size of chloride ions). However, sodium metal is not formed from the electrolysis of concentrated sodium chloride as sodium’s ability to accept electrons is vastly lesser than hydrogen’s. Thus, the difference in electrochemical

discharge potential between these two elements is sufficiently great so as to prevent the discharge of sodium.

Do note that all of the above examples are predicated on the assumption that inert electrodes are used.

Use of non-inert electrodes

When non-inert anodes are used, the anode decomposes in the electrolysis to form metal ions. This is because the free electrons present within the structure of these metals are drawn by the cell out of the anode. Thus, the positive lattice within the metal disintegrates to form mobile positive metallic ions.

Case study: Purification of copper

Copper metal can be purified using the electrolysis of aqueous copper(II) sulfate. In this process, the impure copper metal is attached as the anode.

The copper within the anode decomposes to form copper ions:

Cu (s) Cu⟶ 2+ (aq) + 2 e-

The impurities on the impure copper metal will thus be left at the bottom of the electrolysis tank as the impurities do not participate in the electrolysis reaction.

At the cathode, copper ions are discharged from the solution to form copper metal on the cathode.

Cu2+ (aq) + 2 e- Cu (s)⟶

Thus, pure copper is re-deposited onto the cathode.

Case Study: Copper electroplating

The desired object is attached as the cathode while the electroplating metal (copper) is attached as the anode. A suitable electrolyte that contains the cation of the electroplating metal is used (aqueous copper(II) sulfate).

During electrolysis, the non-inert copper anode will decompose to form copper ions which are discharged into the solution.

Cu (s) Cu⟶ 2+ (aq) + 2 e-

Copper from the solution will also be discharged onto the cathode (desired object), thus electroplating the object.

Cu2+ (aq) + 2 e- Cu (s)⟶

One use of electroplating is the production of electrical contacts. Gold, although being a very desirable material for manufacturing electrical contacts due to its relative inertness and excellent electrical conductivity, is both too expensive and too soft to manufacture entire contacts. Thus, gold is electroplated onto electrical contacts to form a connective layer without experiencing any of the abovementioned side-effects.

Electrical cells

When two metals of differing reactivity are connected while immersed in a conducting solution, the two metals exert an electrical potential onto each other. More reactive metals are more electropositive than less reactive metals. Thus, more reactive metals will “donate” delocalized electrons to less reactive metals, effectively creating a current flow.

The electrical potential difference of a cell (linked to e.m.f) is determined by the difference in reactivity of the two metal electrodes. For example, sodium and magnesium will not have as much potential difference than sodium and copper.

In electrical cells, the more reactive metal is always the anode while the less reactive metal is the cathode.

General rule of electrolysis/electric cell/hydrogen fuel cell

Cathode reactions always reduce while anode reactions always oxidize.